/* 
 * Hawk - A Component Based Test Framework
 *
 * Copyright (C) 2010 http://code.google.com/p/hawk-framework/
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License. 
 */
package org.hawk.applications.finder.android.matching;

public class Correlation {
	
	/**
	 * Returns the correlation between two bytes array
	 *  
	 * @param byte[] scores1 : Answers of the first user 
	 * @param byte[] scores1 : Answers of the second user
	 * 
	 * @return The correlation value -1 until 1
	 */
	public static double getPearsonCorrelation(byte[] scores1, byte[] scores2){
		double result = 0;
		double sum_sq_x = 0;
		double sum_sq_y = 0;
		double sum_coproduct = 0;
		double mean_x = scores1[0];
		double mean_y = scores2[0];
       
		for(int i=2;i<scores1.length+1;i+=1){
			double sweep =Double.valueOf(i-1)/i;
			double delta_x = scores1[i-1]-mean_x;
			double delta_y = scores2[i-1]-mean_y;
			sum_sq_x += delta_x * delta_x * sweep;
			sum_sq_y += delta_y * delta_y * sweep;
			sum_coproduct += delta_x * delta_y * sweep;
			mean_x += delta_x / i;
			mean_y += delta_y / i;
		}
       
		double pop_sd_x = (double) Math.sqrt(sum_sq_x/scores1.length);
		double pop_sd_y = (double) Math.sqrt(sum_sq_y/scores1.length);
		double cov_x_y = sum_coproduct / scores1.length;
       
		result = cov_x_y / (pop_sd_x*pop_sd_y);
       
		return result;
   }
	
}
